Semiconductor components, such as packages, dice and wafers include terminal contacts which provide input/output paths to the integrated circuits contained on the components. For surface mount components, the terminal contacts can comprise solder or gold bumps or balls, bonded to contact pads on the component. For some components, such as chip scale packages, BGA packages, and flip chip devices, the terminal contacts can be arranged in a dense grid array, such as a ball grid array (BGA), or a fine ball grid array (FBGA).
One method for attaching components having this type of terminal contacts into various electronic systems is known as flip chip bonding. With flip chip bonding the terminal contacts on the component are bonded to corresponding substrate contacts on a supporting substrate, such as a module substrate or a printed circuit board (PCB). A typical bonding process involves reflowing terminal contacts made of solder or gold to form metallurgical bonds with the substrate contacts.
One aspect of flip chip mounted components is that thermal stresses can develop at the connections (e.g., solder joints) between the terminal contacts and the substrate contacts. These thermal stresses develop because the components are formed of a first type of material (e.g., silicon), the terminal contacts are formed of a second type of material (e.g., solder or gold), and the substrate is formed of a third type of material (e.g., ceramic or bismaleimide triazine). The different materials have different thermal coefficients of expansion (TCE), such that variations in temperature cause the materials to expand and contract by different amounts.
These thermal stresses can cause fatigue fractures to develop at the connections. The fatigue fractures can affect the reliability of the connections, and the physical bond between the component and the substrate. In addition, fatigue fractures can cause some of the connections to fail entirely. For example, failures from fatigue fractures often occur at the outside corners of a grid array where the stresses are most concentrated.
One method for alleviating the adverse affects of thermal stresses is to encapsulate the terminal contacts in a low stress underfill encapsulant. Typically, the underfill encapsulant also completely fills the space between the component and the substrate. The underfill encapsulant functions to redistribute the thermal stresses over a surface area that is about the same size as the component. In addition, the TCE of the underfill encapsulant can be closely matched to the TCE of the terminal contacts, such that thermal stresses at their connections with the substrate contacts are reduced. The underfill encapsulant also physically bonds the component to the substrate, and protects the terminal contacts from contaminants, such as moisture and dust.
Two different processes have been developed in the art for encapsulating components in an underfill encapsulant. A capillary underfill process is shown in FIG. 1A. A no flow underfill process is shown in FIG. 1B. Either of these processes can be performed at the wafer level on wafer sized components (e.g., semiconductor wafers), or at the die level on die sized components (e.g., semiconductor packages).
Referring to FIG. 1A, the capillary underfill process includes four steps (Steps A-D). Initially, a component 10 and a supporting substrate 12 are provided. The component 10 includes terminal contacts 14, and the supporting substrate 12 includes substrate contacts 16, substantially as previously described. Step A is an alignment step, in which the terminal contacts 14 are aligned with the substrate contacts 16. Step B is a bonding step, such as a solder reflow, in which the terminal contacts 14 are bonded to the substrate contacts 16. Step C is a capillary injection step, in which a dispensing apparatus 18 dispenses viscous underfill material 20 which is drawn by capillary action between the component 10 and the substrate 12. Step D is a curing step, in which the underfill material 20 is cured to formed an underfill layer 22 which encapsulates the terminal contacts 14 and bonds the component 10 to the substrate 12.
As with most processes, the capillary underfill process has certain limitations. For example, voids can form in the underfill layer 22 if the capillary injection step is not performed properly. In addition, the underfill material 20 must have a relatively low viscosity, such that the curing step takes a relatively long time to perform.
Referring to FIG. 1B, the no flow underfill process also includes four steps (Steps 1-4). Initially, in Step 1 the substrate 12 and the substrate contacts 16 are provided. Step 2 is a no flow dispensing step, in which a no flow underfill material 20NF is deposited by a dispensing apparatus 18NF onto the substrate contacts 16 and onto the surface of the substrate 12. The no flow underfill material 20NF has a relatively high viscosity, such that it remains on the area of the substrate 12 on which it is initially deposited. Step 3 is a placement step, in which the terminal contacts 14 on the component 10 are pressed through the no flow underfill material 20NF into contact with the substrate contacts 16. Step 4 is a bonding and curing step, in which the terminal contacts 14 are bonded to the substrate contacts 16, and the no flow underfill material 20NF is cured to form a no flow underfill layer 22NF. Because bonding between the terminal contacts 14 and the substrate contacts 16 typically occurs by solder reflow, the no flow underfill material 20NF is also sometimes referred to as a “reflow” encapsulant.
The no flow underfill process also has certain limitations. For example, the height of the terminal contacts 14 can vary, such that some of the terminal contacts 14 may not physically touch the substrate contacts 16 during the placement step. These terminal contacts 14 may not bond properly to the substrate contacts 16 affecting the physical and electrical connections therebetween. Similarly, the surface of the substrate 12 may be non planar causing the same bonding problem. Also, the no flow underfill material 20NF is difficult to formulate with the required physical properties. For example, polymers with no flow characteristics may not have a good TCE match with the terminal contacts 14, and may have low modulus of elasticity characteristics. Because of these characteristics, no flow underfill layers 20NF fail earlier in temperature cycling tests than capillary underfill layers.
Referring to FIG. 1C, another problem associated with no flow underfill processes is illustrated. In this example the terminal contacts 14 comprise a solder material, such as a SnPb solder, that has been reflow bonded to the substrate contacts 16. In addition, the no flow underfill layer 22NF includes a filler which includes non conductive particles 27. For example, the non-conductive particles 27 can comprise silicates configured to reduce the TCE, and to adjust the viscosity of the underfill layer 22NF. As shown in FIG. 1C, some of the non-conductive particles 27 can be trapped at the interface of the terminal contacts 14 with the substrate contacts 16. These trapped non-conductive particles 27 can add resistance to the electrical connections between the terminal contacts 14 and the substrate contacts 16. In addition, these trapped non-conductive particles 27 can adversely affect the physical bond between the terminal contacts 14 and the substrate contacts 16.
Referring to FIG. 1D, another problem associated with no flow underfill processes is illustrated. In this example the terminal contacts 14 are made of gold, and have again been bonded to the substrate contacts 16. In addition, the no flow underfill layer 22NF includes Ni particles 29. As before, some of the Ni particles 29 can be trapped at the interface of the terminal contacts 14 with the substrate contacts 16. These Ni particles 29 have a different resistivity than the gold terminal contacts 14, such that the resistance of the electrical connections between the terminal contacts 14 and the substrate contacts 16 can vary, and change abruptly across the terminal contacts 14. In addition, the Ni particles 29 can adversely affect the physical bond between the terminal contacts 14 and the substrate contacts 16.
The present invention provides a no flow underfill material, and a method of underfilling, that overcome some of the limitations of conventional underfill processes.